A 39-year-old female had cor triatriatum (CT) detected as an incidental finding on transthoracic echocardiography performed to evaluate chest pain. By conventional two- and real time three-dimensional transesophageal echocardiography, the CT membrane had a communicating orifice connecting the accessory and main left atrial chambers that measured 1.3 × 0.8 cm. The resting mean transmembrane gradient was 2 mm Hg. The postexercise mean transmembrane gradient and pulmonary artery pressure were 6 and 40 mm Hg. Extrapolating from cutoff values for postexercise gradients and pulmonary pressures in patients with mitral stenosis, we advised deferring surgery and close clinical and echocardiographic follow up.